Superconducting Quantum Interference Devices (SQUIDs) are comprised of tiny loops of superconducting material in which Josephson junctions are placed in the loop path. A Josephson junction is a region of material that provides a weak link between two fully superconducting regions. The DC SQUID has two symmetrical Josephson junctions. They are able to sense extremely small magnetic fields. Non-uniforms arrays of DC SQUIDs and DC bi-SQUIDs, which are DC SQUIDs with an additional Josephson junction bisecting the superconducting loop, have been modeled in different array designs and coupling schemes to determine their linearity and sensing capacities and have been fabricated in low temperature superconducting materials. A SQUID-based sensor detects minute magnetic fields and is decoupled from the size of the signals wavelength. Hence the device can sense signals in the DC-GHz range, but still be contained fully on a ˜1×1 cm chip.
While it is possible to electronically subtract the (voltage) outputs of SQUID (and by inference SQUID Array) magnetometers (aka an extrinsic gradiometer), there are limitations due to dynamic range, mismatches in gain, etc. that limit the efficacy of these techniques. There is a need for an improved gradiometer.